Multi-phase linear electromagnetic motors are generally made up of a row of permanent magnets. Induction coils or phase coils are arranged facing the magnets and in directions that are transverse to the flux generated by the magnets. A typical multi-phase linear motor for driving a motion stage may have two or three coils arranged longitudinally, in such a way as to be arranged in line in the direction of the motion stage. Hence, the coils are arranged parallel to the motion direction. In a two-phase motor where there are two sets of phase coils, the two phase currents are mutually offset by 90°. In a three-phase motor, the three phase currents are mutually offset by 120°.
A multi-phase linear motor may drive motion along a chosen motion axis. Heat is generated during operation of the linear motor when a driving current passes through it, which raises the temperature of the motion stage. On the other hand, when there is no motion, the temperature of the motion stage drops since the large driving current of the motor is absent. The variance in the temperature of the motion stage is undesirable in high precision applications. It is essential to maintain thermal stability of the motion stage for such applications to attain high positioning accuracy. A number of techniques are used in the prior art to control and maintain the thermal stability of the motion stage. Some applications use materials with low coefficients of thermal expansion such as invar, so that the expansion of the material due to an increase in temperature is low. Elaborate thermal compensation circuits can also be used, which include heaters for keeping the linear motor in the motion axis at a constant temperature.
There are however disadvantages in using the aforesaid methods to regulate the temperature of the motion stage. Motion stages which use materials having low coefficients of thermal expansion such as invar are not only costly but make the motion stage heavy. Motion stages that use thermal heaters or cooling systems for compensation are complicated, compromise reliability of the motors and increase expenses even further. It is therefore desirable to devise a compact and low cost method for maintaining the thermal stability of motion stages.